Blender Git Loki

Cycles: Improve volume stack size calculation

Only count volume objects after shader optimization.

Allows to discard objects which don't have effective volume
BSDF connected to the shader output (i.e. constant folded,
or non-volume BSDF used by mistake).

Solves memory regression reported in T92014.

There is still possibility to improve memory even further
for cases when there are a lot of non-intersecting volume
objects, but that requires a deeper refactor of update
process. Will happen as a followup development.

Differential Revision: https://developer.blender.org/D12797